Munitions material released into the environment as low-order detonation debris or breached unexploded ordnance (UXO) represents a long-term source of explosives and associated contaminants on training ranges and battlefields. Dissolution and transport of munitions-associated compounds into the soil and subsurface pose a potential environmental risk and would dramatically escalate any restoration costs.
The primary objective of this proof-of-principle investigation was to quantify the effectiveness of multiple additives designed to promote explosives degradation upon exposure to environmental conditions. The primary working hypothesis was that small amounts of non-explosive amendments will at least initiate degradation of RDX and TNT upon exposure to light and/or water. Self-remediation must be achieved without substantial deleterious impact on munitions performance, safety, or storage, thereby presenting a severe constraint on additive selection and proportion.
The overall approach involves the systematic preparation and testing of a suite of munitions formulations containing alternative inorganic amendments at low volume fractions (≤5%). Multiple concentrations of metal or oxide additives were mixed into standard PBXN-107 formulations. Preparations yielded replicate tablets containing 0, 5, 10, and 15 weight-% of three separate additives – Fe, FeNi (4Fe:1Ni), and doped anatase. Composition B formulations were prepared by amending an existing Composition B with 5 volume-percent of additives. Samples of neat and amended formulations were subjected to a suite of sensitivity tests. Assessments of additive stability in the PBXN mixture are required to assure safe handling during preparation, shipment, and subsequent environmental testing.
Self-remediation experiments were conducted on the modified formulations. Crystalline explosive samples of known initial mass and composition were placed on or within a short column of packed sand and flushed intermittently with a uniform amount of deionized water. Columns were kept either in a dark or lighted, closed chamber between wetting events. Gravity drainage was captured in an amber glass vial over several days and analyzed for explosives.
The amendments evaluated in PBXN-107 (Fe, FeNi, and TiO2) and Composition-B (Fe, TiO2, and dithionite) do not appear to be effective candidates for explosives self-remediation formulations. TNT degradation products attributable to the amendments were detected at low levels while the vast majority of dissolved TNT and RDX remained intact.
All three amendments to PBXN-107 interfered with the curing of the acrylic binder. As a result, all samples were friable to varying degrees, generally in proportion to the amount of amendment. The compromised condition precludes meaningful comparison among replicates and between treatments. However, samples were deemed adequate to test for potential enhanced RDX reactivity with amendments. No RDX reaction products were indicated by standard HPLC analysis methods. Whether and how the acrylic binder in PBXN-107 can be modified to accommodate particulate self-remediation amendments is unknown.
Impact tests reveal a reduction in drop height for amended PBXN-107, suggestive of increased sensitivity. However, no reactions were detected in BAM and ABL friction tests or in the electrostatic tests with PBXN. Composition B samples were not subjected to sensitivity tests. Uniform masses of Composition B were amended with 5 volume-percent of three separate amendments with no detectable interference with sample cohesion. Chemical reduction of TNT to the 4-amino-DNT intermediate was observed with the dithionite amendment and, to a lesser degree, with the powdered iron amendment. A small increase in TNB (1,3,5-trinitrobenzene), a common photodegradation product, was observed with Comp-B amended with the anatase photocatalyst.
An exploratory test of an unencapsulated, powdered NaOH proved too reactive to evaluate for self-remediation potential. Encapsulated NaOH may have potential as an amendment designed to degrade detonation properties. Powdered Fe amendments to the Comp-B (5 vol%) and PBXN (5 wt% ≈ 1 v%) imparts a magnetic property that might be exploitable in range cleanup.
This investigation is the first to explore alternative approaches to explosives self-remediation by incorporating reactive amendments. As such, regardless of the disappointing outcome, it will direct future development along more fruitful paths. Lessons learned will guide the selection of alternative amendments, target munitions, experimental procedures, and realistic goals. Self-remediation and munitions deterioration remain valuable goals worth pursuing.
Some of the severe challenges involved are now defined more clearly. The most severe constraint of very small permissible volumes for any non-energetic amendment may prove to be an insurmountable challenge. Although only low levels of degradation were indicated in some of the experiments, alternative amendments may prove more successful. Many potential reactants, photocatalysts, and perhaps microbes remain to be evaluated as self-remediation amendments.
Elucidation of why any particular self-remediation amendment is effective (or not) should be considered in future experimental designs. A more detailed characterization of the materials at all stages of exposure could provide essential insights. Examination by scanning electron microscopy (SEM), optical petrography, and X-ray diffraction (XRD), among other tools, should provide insights as to the nature of environmental weathering, particularly with respect to the condition and fate of amendments.